- The injury due to the coffee burn was due entirely to the elevated temperature.

- The machine would not have served at the scalding temperature had McDonald's not made the modification.

- The modification was specifically made to raise the temperature to a level that turned out to be unsafe.

The question of whether a modification was made would almost certainly come out in court because a McDonald's employee would be unlikely to perjure themselves over it.

Therefore, all the liability would be on McDonald's.

Though I do see your point about legal costs being incurred even if the case of Kuering being completely innocent. It seems that having a culture instilled with a nebulous concept of "liability" encourages people to limit other's ability to take their own risks.

(Edit: formatting)